1. Field of the Invention
The invention relates to spatial light modulators and more specifically to writing such modulation at very low light levels by use of an avalanche photodiode (APD) structure to promote electrical avalanching and increase the sensitivity of the device.
2. Description of the Related Art
Many types of spatial light modulators are in use at the present time. Such devices may be addressed optically, electrically or mechanically as means to impose modulation on an incident wavefront that reads such modulation. These devices may modulate the phase, the amplitude, or both the phase and amplitude of the reading wavefront. Such devices may either reflect or transmit the modulated wavefront.
Many spatial light modulators utilize silicon and liquid crystal materials wherein various optical effects such as the electro-optic effect, the thermo-optic effect and the photorefractive effect are used for modulation of light. These effects may be enhanced by the application of external electric fields to the material that modulates the reading light pulse. The electric fields, for example, may enhance the response of the material through the action of electron avalanches. In existing spatial light modulators, such electron avalanches are created in a first component and elicit an optical modulation in a second component. Such a first component, if used, comprises a microchannel plate or a photomultiplier tube.
Many spatial light modulators require the fabrication of definite pixels in order to create localized modulation of light. This fabrication process increases the expense of such devices.
The use of electron avalanches have been demonstrated to create an observable optical response in an avalanching material (See S. M. Horbatuck, D. F. Prelewitz and T. G. Brown, "Avalanche Enhancement of Optical Nonlinearities in Semiconductor Junctions" Applied Physics Lett., Vol. 56 (24), pp. 2387-2389 (1990): see also J. H. Swoger and S. J. Kovacic, "Enhanced Luminescence due to Impact Ionization in Photodiodes" J. Appl. Phys. Vol. 74(4), pp 2565-2571 (1993).)
The extension of the above work to low-cost spatial light modulators operating at very low light levels, high resolution, and fast response requires additional electrical and optical techniques. To reduce the cost of spatial light modulators, it is desirable to eliminate need for fabrication of pixels and for a separate electron-avalanching component. This is achievable by use of electron avalanches within the same material that modulates the light. This eliminates the need for a separate avalanching component; it also eliminates the need for pixellation if the avalanching remains localized to the point of excitation. If the external electrical circuit is properly designed, such localization does in fact occur.
Very large avalanches may also be generated to greatly enhance an optical response. This is achieved when certain APD structures are electrically biased above the breakdown voltage. This mode of electrical operation, referred to as the Geiger mode, can generate tens of millions of photoelectrons subsequent to the absorption of a single photon. Operation in this mode is low in noise and uniform in response if a high-quality APD structure is utilized, and if the external circuit has low inductance and intermediate resistance.
Under proper conditions, when the electrical response is strong and localized, the optical response is also strong and localized. If the optical response depends on the thermo-optic effect, then the reading pulse must not be delayed by too much time after the writing avalanche ceases--otherwise the avalanche induced heating will dissipate and diffuse. Similarly, if the optical response depends on the photorefractive effect, then the delay must be short to avoid diffusion of vacated carrier traps.
Another difficulty occurs when using an avalanche material for the optical response: undesirable avalanches may be created by the reading pulse, instead of or in addition to those created by the writing pulse. Such avalanches are avoidable using several techniques. First the wavelength of the reading light may be at a longer wavelength than that of the writing light, so that the energy of the photon of the reading light is less than the band-gap energy of the avalanche material, and is therefore not absorbed. Second, the external bias of the circuit can be actively quenched so that negligible breakdown occurs when the reading light is incident. This latter approach obviously requires a delay between the writing pulse and the reading pulse, a delay which cannot be too long based on the remarks of the preceding paragraph.